1. Field of the Invention
This invention relates generally to navigational signal receivers. More particularly, it relates to a new baseband integrated circuit architecture for direct sequence spread spectrum (DSSS) communication receivers.
2. Description of the Related Art
Satellite-based radio navigation systems have become widely adopted in many commercial and military applications. Exemplary systems in operation or development include the NAVigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS), the Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), an European satellite navigation system called GALILEO, the wide area augmentation system (WAAS), and the local area augmentation system (LAAS). These systems permit a user with an appropriate direct sequence spread spectrum (DSSS) signal receiver to determine his or her position with respect to the Earth. Direct Sequence Spread Spectrum is a modulation technique where a pseudorandom noise sequence directly phase modulates a data-modulated carrier. The DSSS signal has a noise-like spectrum and appears to be noise to all but the intended receiver
As an example, the GPS constellation has 24 operational satellites. These satellites are positioned in six different orbital planes such that at any time a minimum of six and a maximum of eleven satellites are visible to any user on the surface of the Earth, except in the polar region. The satellites operate in near circular 20,200 km (10,900 nm, or about 12,000 miles) orbits at an inclination angle of 55 degrees and with approximately a 12-hour period.
Each satellite contains at least one atomic clock and transmits a navigation message that contains an accurate system time and its orbital position referenced to the atomic clock. The navigation message also contains clock behavior, status messages, and correction data such as ionospheric delay, time offset, etc. An almanac that gives the approximate data for each active satellite is also provided.
Each satellite transmits on two L-band frequencies: L1=1575.42 MHz and L2=1227.6 MHz. Three sets of pseudorandom noise (PRN or PN) ranging codes are in use: the coarse/acquisition (C/A) code, the precision (P) code, and the Y-code.
The C/A code set, also known as Gold code, has a 1.023 MHz chip rate. In spread spectrum technology, the term “chip” refers to a single bit of a pseudorandom sequence (PN-sequence) and the term “chip rate” refers to the rate at which bits of a PN-sequence are shifted. The Gold code therefore has a length of 1023. The term “code” refers to the binary bit stream (the pseudorandom sequence) used to spread a signal over a wide range of frequencies for transmission. This spreading improves the accuracy of position estimation. Other advantages include interference rejection and low spectral power density, i.e., the power level at a given frequency.
A correlator at a receiver despreads this signal to the original data bandwidth by correlating it with a locally generated PN-sequence identical to and in synchronization with the PN-sequence used to spread the carrier at the radio transmitter, e.g., a GPS satellite vehicle (SV). Typically, this dispreading occurs after the signal received at the antenna has been amplified and down-converted to a suitable carrier frequency, also known as the intermediate frequency (IF). The hardware section associated with the amplification, down-conversion, and analog-to-digital conversion (ADC) is called the radio frequency (RF) stage. The other section, which processes the RF stage output and generates the position, velocity, and time information, is called the baseband (BB) stage.
The sampling rate at the BB stage can be any multiple of the PN code rate. A minimum of two samples per chip (bit) is needed, which results in a minimum sampling rate of 2.046 MHz. The sampled signals are then made available in two channels, one in-phase (I) and the other in-quadrature (Q). The resulting signals are then correlated with the locally generated PN code. The local code generator is driven by a code Numerically Controlled Oscillator (NCO). The result of the correlation is sent to a processor and further processed to determine the code and carrier phase offset. The processor sends a control signal to the code NCO and the carrier NCO so that they are in alignment with the input (sampled) signal. When the incoming signal is aligned with the locally generated PN code and carrier, the data bits in the signal can be extracted. The extracted data are used in computing the satellite position and hence the receiver's position, velocity, etc.
U.S. Pat. No. 6,845,124, issued to Mattos et al., discloses a GPS receiver integrated circuit. The hardware complexity is high as this IC has 16 hardware channels. U.S. Pat. No. 6,067,328, issued to Lewellen et al., discloses a GPS receiver with a baseband detector. The baseband detector includes a NCO, but does not provide all different clock frequencies required. These and other existing baseband architectures usually contain a large number of hardware correlators. The gate count, which affects power consumption, efficiency, and performance, is therefore correspondingly high. Furthermore, in conventional baseband architectures, the NCOs are usually located off the baseband IC chip and do not adjust to the different final IF frequencies.
Clearly, there is a need in the art for a new baseband IC chip with an optimal hardware architecture that minimizes gate count, thereby increasing power efficiency and performance. The present invention addresses this need.